Multiple-Step Humidity-Induced Single-Crystal to Single-Crystal Transformations of a Cobalt Phosphonate: Structural and Proton Conductivity Studies

Humidity-induced multiple-step single-crystal to single-crystal (SC-SC) transformations are observed in the cobalt phosphonate (NH4)3[Co2(bamdpH)2(HCOO)­(H2O)2] (1), where bamdpH4 is (benzylazanediyl)­bis­(methylene)­diphosphonic acid, [C6H5CH2N­(CH2PO3H2)2]. Under high-humidity conditions (95% RH), compound 1 experiences hydrolysis at 60 °C which is accompanied by the transformation from a double-chain structure of compound 1 into a single-chain structure of [Co­(bamdpH2)­(H2O)2]·2H2O (2). When the humidity is below 10% RH, part of the lattice water in compound 2 can be released, forming a third phase, [Co­(bamdpH2)­(H2O)2]·H2O (3). The structural transformation processes have been monitored by infrared and proton conductivity measurements.